1. Field of the Invention
The present invention relates to a method for producing a semiconductor device through the laser lift-off process.
The method of the present invention is very useful for improvement of quality and productivity of semiconductor devices.
2. Background Art
Conventionally known techniques for producing a semiconductor wafer through the laser lift-off process include a technique disclosed in the specification of U.S. Pat. No. 6,071,795. In the disclosed technique, a GaN crystal layer grown on a surface of a sapphire substrate is bonded to a silicon (Si) supporting substrate by use of an electrically conductive adhesive, and subsequently the GaN crystal layer is irradiated with laser beam from the back surface (i.e., the surface opposite the crystal growth surface) of the sapphire substrate, to thereby melt crystalline GaN grown in the vicinity of the crystal growth surface of the sapphire substrate, for removal of the sapphire substrate from the GaN crystal layer of interest.
Through such a production process (laser lift-off process), the non-electrically conductive sapphire substrate can be removed from the GaN crystal layer of interest without causing any damage to the GaN crystal layer. Therefore, when appropriate electrical conductivity is imparted to the supporting substrate, electrodes can be formed on both surfaces of a semiconductor wafer formed of the semiconductor crystal layer, which has been grown on the sapphire substrate.
In the case of production of an LED or a similar device, in which light is extracted through the back surface of the semiconductor crystal layer on which the sapphire substrate has been provided, removal of the sapphire substrate through such a production process is advantageous in that, for example, light extraction efficiency is improved.
The aforementioned electrically conductive adhesive employed for bonding between the GaN crystal layer and the supporting substrate is generally formed of a solder material containing a metal (e.g., AuSn). Therefore, when separation trenches are formed, by means of a dicer, in the semiconductor wafer before the wafer is cut into individual chips, the metal causes clogging of the dicer. Meanwhile, a layer formed of the metal is difficult to break as compared with a layer formed of a material other than the metal (e.g., a semiconductor crystal), and thus difficulty may be encountered in reliably cutting the semiconductor wafer into individual chips.
In view of the foregoing, the present inventors have conceived a method in which a metal layer formed of, for example, an electrically conductive adhesive is not provided on at least a region through which a dicer blade is to be positioned.
FIG. 4A shows the configuration of a semiconductor wafer in which a metal layer formed of an electrically conductive adhesive, an electrode, or the like and employed for bonding between the aforementioned GaN crystal layer and the supporting substrate is provided in a selective manner; i.e., in a dicer-blade-positioning region and in the vicinity thereof, no metal layer is formed. The semiconductor wafer A1 includes a sapphire substrate 1, and a semiconductor crystal layer 2 grown on the crystal growth surface of the sapphire substrate 1. Islands-patterned electrodes 4 formed on a surface 2b of the semiconductor crystal layer 2 are bonded to islands-patterned electrodes 5 formed on a surface 3a of a supporting substrate 3 by use of an electrically conductive adhesive constituting the uppermost layer of at least one of the electrodes 4 and 5.
Thus, portions where the metal layer is provided (spaces R) are provided in the semiconductor wafer so that semiconductor chips are periodically formed. With this configuration, when separation trenches for separating the semiconductor wafer into individual chips along separation planes σ are formed on a to-be-exposed surface 2a of the semiconductor crystal layer 2, even if the tip of a dicer blade reaches the vicinity of a bonding portion between the electrodes 4 and 5, the dicer does not come into contact with the metal layer. Therefore, clogging of the dicer caused by the metal can be obviated. In addition, since the laterally provided metal layer is not shared with individual chips to be formed, the semiconductor wafer can be more reliably separated into chips.
However, the present inventors have found that this method raises the following problems. When the sapphire substrate 1 is removed through the laser lift-off process, as shown in FIG. 4B (i.e., semiconductor wafer cross-sectional view), cracks γ are prone to be formed in the semiconductor crystal layer 2 along the aforementioned spaces R. Such breakages (cracks γ) cause difficulty in neatly providing separation trenches for a to-be-formed device at predetermined positions. These cracks also tend to cause problems in the subsequent step (e.g., incomplete separation), resulting in reduction of the yield of a target semiconductor device.
Easy occurrence of such cracks γ during the course of lift-off of the sapphire substrate 1 through laser beam radiation is considered to be due to the following two causes.
(Cause 1) When a portion of the semiconductor crystal layer 2 is melted in the vicinity of the interface between the layer 2 and the sapphire substrate 1, a heat radiation path is partially blocked by the spaces R. Therefore, the temperature of the molten portion increases locally, and stress due to thermal expansion is difficult to reduce by heat radiation.
(Cause 2) Stress due to thermal expansion is concentrated at a structurally weak portion which has been formed through provision of the spaces R, and thus breakages (cracks γ) tend to occur at the structurally weak portion.